How do I see compiler errors and what's wrong with this?

Trying to write a function that converts the info.month from the abbreviated month name to a number. So got this function...

function getMonth(mm)
{
var m;

if(mm.toString() == "Jan")
{
m = 1;
}
return m;
}

And call it like this...

getMonth(info.month)

Why doesn't it return 1?